The support structure and the filter bag may form part of a bag filter in a filter unit or of a filter integrated in process equipment such as e.g. spray dryers and fluid bed processing equipment. Such bag filters are well known in the prior art and are adapted to separate particulate matter from a gas. One example of use of the bag filter integrated in the process equipment is shown in U.S. Pat. No. 6,463,675 (Assignee: Niro A/S). Bag filters forming part of a filter unit are disclosed in, i.a., U.S. Pat. Nos. 6,676,720, 6,149,716 and 6,332,902 (Assignee of all patents: Niro A/S).
The filter bags may be of any material suitable to the purpose, and may for instance be made of felt or woven of e.g. polyester or Teflon® (PTFE). A felt or polyester material may be permeable to the cleaning fluid, while Teflon® is not. Alternatively, a filter bag may be manufactured from a permeable material and subsequently coated with an impermeable material, e.g. a polyester material coated with Teflon®. In general, the material used in such filter bags is flexible.
Traditionally, such a filter bag is, on its inside, supported by a support structure in the form of a basket to prevent the flexible bag to collapse due to the process gas flow from the outside to the inside of the bag. Such a support structure traditionally includes a number of threads or rods extending in the longitudinal direction of the filter bag and spaced from each other, seen in the circumferential direction, in suitable intervals. The longitudinally extending rods are held by a number of rings or annular rods that are spaced from each other, seen in the longitudinal direction, in suitable intervals. Most often, the filter bag is positioned with the longitudinal direction extending substantially vertically in the mounted position, e.g. when the top of the filter bag is mounted in the filter unit or in the process equipment. Hence, in such a position the longitudinally extending rods are substantially vertical and the annular rods are substantially horizontal.
In between periods of operation the filter unit is cleaned, e.g. by a cleaning-in-place process (CIP) involving cleaning nozzles located at least in the clean gas chamber at the upper outlet side of the bag filters. The cleaning nozzles are supplied with a cleaning fluid that possibly includes a cleaning agent. It is also possible to supply the cleaning nozzles with gas pulses in combination with the supply of cleaning fluid. During cleaning the cleaning fluid is washed down into the bag filters together with any entrained particles or powder from the clean gas side. Similarly, the bag filters are cleaned on the outside.
A great demand on the quality of the CIP exists. In the chemical industry, e.g. for dyes, it is essential to avoid cross-contamination when shifting from one colour to another. In the dairy and food industry, the quality of the cleaning is very important for bacteriological reasons. Furthermore, the cleaning quality is essential in the pharmaceutical industry, again for bacteriological and health reasons and to meet requirements of authority regulations.
In the pharmaceutical industry, the active components may not escape from the process equipment and therefore, impermeable filter bags may be prescribed for this area of application. In a CIP process, the filter bags are usually cleaned from the outside, i.e. the product side, and from the inside, i.e. the clean gas side. In order to drain off the cleaning fluid from inside the filter, a controlled valve may be provided in the bottom of the filter, in particular in the case when the bottom is impermeable. A prior art valve of this kind is e.g. disclosed in U.S. Pat. No. 5,444,892 (Assignee: Niro-Aeromatic AG).
Particularly within this field of application, but also in other fields, there is a need for inspecting and controlling the operative position of the valve from the outside of the filter unit or processing equipment in order to detect whether the valve is open or closed, and to activate and de-activate the valve.
Also for cleaning considerations, it is vital that there are as few items as possible connected to the filter. Furthermore, the items must be easily cleanable. In addition to the fact that the prior art support structure may suffer from some disadvantages with respect to sufficient cleaning due to the configuration of the rods, the filter bag material is exposed to wear. This is due to the fact that the filter bag material is mounted such that it abuts on the support structure, i.e. on the longitudinally extending rods and the annular rods. During operation, process gas is blown through the filter bag, thereby forcing the filter bag material against the support structure. Furthermore, the filter bag is subjected to pressure pulses ejected from a suitable supply to shake off product adhering to the outside of the filter bag, at intervals during filtration. Consequently, the filter bag material is exposed alternately to circumferential tensile stress and to local tension at the lines of contact between the filter bag material and the rods.